What Is the Stair Tread Calculator?
Calculate the code tread depth of your stairs from the two numbers you can actually measure: the tread board depth and the nosing projection. Three different measurements get mixed up on stair projects, and this calculator keeps them straight. The unit run is the horizontal distance between riser faces. The tread board depth is the full front-to-back depth of the board, which equals the unit run plus the nosing projection (the part of the tread that overhangs the riser below). The code tread depth is the horizontal distance from one nosing to the next, which works out to exactly the unit run. The IRC (R311.7.5.2) regulates that code depth with a 10-inch minimum, and it requires the nosing projection itself to fall between 3/4 inch and 1-1/4 inches. So the math the calculator runs is simple but easy to get backwards: code tread depth equals tread board depth minus nosing projection. Enter a 12-inch board with a 1-inch nosing and you have an 11-inch code depth, comfortably legal. Enter an 11-inch board with the same nosing and you are at exactly 10 inches, the minimum. This matters because tread stock is sold by board depth while inspectors measure nosing to nosing, and the translation between the two is where most tread layouts quietly fail.
How to Use This Calculator
- Measure the tread board depth, the full front-to-back depth of one tread board, and enter it.
- Enter the nosing projection, the distance the tread front overhangs the riser face below. Code allows 3/4 inch to 1-1/4 inches.
- Review the derived code tread depth: board depth minus nosing projection. This is the number inspectors measure nosing to nosing.
- Compare the result against the 10-inch IRC minimum. If you are under, the calculator shows how much deeper the board or how much smaller the nosing must be.
- Check the nosing projection itself against the 3/4 to 1-1/4 inch band. A projection outside that range fails even when the depth math works.
- Run the check for every step in the flight. The code also requires the largest tread depth to be within 3/8 inch of the smallest, so uniformity matters as much as the minimum.
